Excretion and Osmoregulation
Outline
• Introduction
• Comparative physiology of osmotic
regulation
• Mammalian kidney
• Evolution of the vertebrate kidney
All organisms have an excretory
system of some type in order to
• Manage solutes in body fluids
• Manage water content of the body
• Remove metabolic end-products
• Remove foreign substances
Two basic modes of excretion
• Ultrafiltration- pressure filtrate of blood,
withholds protein and large solutes but
water and small solutes pass
• Active transport- against a concentration
gradient. If directed away from the
organism, secretion. If directed toward
the organism, reabsorption.
Three functions of excretory
systems
• Ultrafiltration
• Secretion
• Reabsorption
Osmotic Environments
• Aquatic environments (71% of
Earth’s surface)
– Sea waters- 3.5% salts
– Mediterranean – 4% salts
– Saturated with salts• Great salt lake (NaCl)- no fish, some shrimp
• Dead Sea (MgCl2)
Osmoconformers and
osmoregulators
• Osmoconformer- change osmotic
•
•
•
concentration of body fluids to match
environment
Osmoregulator- regulate composition of
body fluids regardless of environment
Seawater-1000 mOsM- animals risk losing
water to the environment
Fresh water-animals risk taking on too
much water and losing salt
Two major categories of
resistance to changes in
osmotic environment
• Stenohaline –limited tolerance to changes
in osmotic composition of environmentmost fish are stenohaline
• Euryhaline- can tolerate wide fluctuations
in environmental osmotic concentrations
Marine vertebrates
• Elasmobranchs, hagfish, crab-eating
frogs
– Osmolarity of body fluids equals that of
sea-water, i.e. 1000 mOsM
• Lampreys, teleost fishes
– Osmolarity of body fluids is one-third
that of seawater, approx. 300 mOsM.
Marine Elasmobranchs
• Maintain salt at one-third that of
seawater, but add organic urea to
body fluids-100 times greater than
mammals
• Total body fluid osmolarity is close to
seawater
• Produce trimethylamine oxide
(TMAO)- neutralizes toxic effects of
urea
• Shark proteins including enzymes are
dependent upon urea
• Excess Na+ excreted by ‘rectal gland’
urea
TMAO
Teleosts
• All teleosts maintain body salt
concentration at 1/3 that of seawater
• Marine teleosts must drink water and get
rid of excess ions from water
• Ion excretion (Na, Cl) is performed by gills
• Mg, SO4, and other divalent ions secreted
by kidneys
Saltwater
http://www.itresourcing.com.au/aquaculture/species/images/
Freshwater
http://www.itresourcing.com.au/aquaculture/species/images/
Amphibians
• Risk losing ions to freshwater
• Actively reabsorb Na and Cl across skin
Terrestrial vertebrates
• Face dehydration from any body surface
• Birds and reptiles produce uric acid to
conserve water
• Develop skin which prevents water loss
Dipodomys microps, the Kangaroo rat
Kangaroo rats
• Abundant in deserts
• Do not drink water
• If given a diet of barley or oats, body
•
weight remains the same for months
Water (g) formed per gram of substrate:
– Starch = 0.56
– Fat=1.07
– Protein=0.45
Nasal turbinates
• Increase water reabsorption from exhaled
air
• Nasal turbinates reduce temperature of
exhaled air
• More water vapor condenses on nasal
epithelium during exhalation
Phylogeny of osmoregulatory
and excretory organs
Contractile vacuole in Paramecium
• Found only in freshwater organisms
• Not a true excretory organ because it does
not produce an ultrafiltrate
• Excess water is channeled to vacuole,
vacuole swells
• Water expelled through pore
Paramecium- contractile
vacuole
Protonephridia
• Blind ended excretory organ
• Solenocyte (1 flagella), flame cell (many
flagella)
• Found in flatworms, rotifers
• May or may not produce an ultrafiltrate
• Since there is no circulatory system in
these animals, flame cells must be located
throughout the body
http://www.cartage.org.lb/en/themes/
Sciences/Lifescience/GeneralBiology/P
hysiology/ExcretorySystem/Invertebrat
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Metanephridia
• Only in eucoelomate animals
• Filters fluid from coelom
• Produces an ultrafiltrate
Earthworm
Malpighian tubules
• Found in insects
• KCl and NaCl transported from coelomic
fluid into Malpighian tubules
• Initial urine formed in tubules
• Final urine formed in rectum
• Produces a concentrated urine but not an
ultrafiltrate
How it works
• K+ actively transported into M. tubule,
conc. 30 times body fluid
• Passive Cl diffusion into M. tubule
• Water follows ions
• In hind gut, water is reabsorbed, uric acid
precipitates
Direction of blood flow
• Afferent arteriole
• Glomerulus
• Efferent arteriole
• Vasa recta
• Vein
Function of nephron
• Glomerular filtration (urea, glucose,
Na, K, Cl)-no proteins
–Same proportion of ions, solutes
and water in glomerulus as in blood
plasma
–No blood cells filtered
–Filtration based upon molecular
size
Function of nephron
• Tubular reabsorption
– 99.9% of water
– Most salts
– Most reabsorption by the proximal tubules
Function of the nephron
• Tubular synthesis
– Some amino acids are deaminated
• Tubular secretion
– Regulates blood levels of K, H+ and HCO3
Brush border in
proximal
convoluted
tubule
Glomerular filtration
• 125 mL/min, about 200 L/day
• Affected by
– Net hydrostatic pressure difference between
capillary and Bowman’s capsule-favors
filtration
– Colloid osmotic pressure of blood, opposes
filtration
– Hydraulic permeability-sieve like properties of
the filtration barrier
• Capillary endothelium
• Basement membrane
• Bowman’s capsule
3
2
1
Calculating GFR
• Patient injected with inulin
• After a while, plasma sample drawn and
inulin concentration measured in plasma
• Concentration of inulin measured in urine
• Urine volume determined
• Since all inulin removed, GFR equals inulin
clearance
Glomerular Filtration rate
• GFR = VU/P
• V = urine volume, mL/min
• U = urine inulin, g/mL
• P = plasma inulin, g/mL
• GFR, mL/min
• Inulin is freely filtered, not secreted, not
reabsorbed
Tubular reabsorption
• 200 L (50 gallons) filtered, 1 L urine
produced per day, 99% of water
reabsorbed
• 99% of sodium reabsorbed
• Plasma glucose clearance = 0
• Transport maximum for glucose = 365
mg/min
Proximal tubule
• Reabsorbs 67% of Na in lumen
• All glucose reabsorbed
• Water follows passively
• 66-75% of filtrate reabsorbed before loop
of Henle
• At the end of the proximal tubule, fluid is
isosmotic even though Na and water
reabsorbed
Concentrating mechanism
• As fluid moves from proximal tubule to
descending loop of Henle it is isosmotic
with ECF
Descending loop of Henle
• Not permebale to NaCl, urea
• Permeable to water
Concentrating mechanism
• In descending loop, water moves out of
tubule because descending loop is
permeable to water
Ascending limb of the loop of Henle
• No transport of NaCl in thin limb
• Permeable to NaCl, passive diffusion
• Impermeable to water, urea
Concentrating mechanism
• As fluid moves up ascending limb,
– NaCl moves out from fluid passively in thin
segment
– NaCl is transported out in thick segment
– No movement of water here as impermeable
to water
Medullary thick limb
• Active transport of NaCl from lumen to
ECF
• Impermeable to water
• Because of NaCl transport, urine is slightly
hypo-osmotic here
Distal tubule
• Transport of K+, H+, NH3+ into lumen
• Transports Na, Cl and HCO3- out of lumen
• Permeable to water, water follows NaCl
Collecting duct
• Permeable to water. Water leaves urine
for higher osmotic gradient in extracellular
fluid.
• Permeable to urea at the distal end
• Permeability to water is under hormonal
control be ADH
Concentrating mechanism
• Only birds and mammals are able to
produce a concentrated urine
• Only birds and mammals have a loop of
Henle
• Desert mammals have longer loops of
Henle
1. Active transport of
NaCl out of ascending
thick limb and distal
tubule
2. Water follows osmotic
gradient and leaves
distal tubules and
descending limb
3. Urea becomes more
concentrated- the only
part of collecting duct
that is permeable to
urea is medullary
portion
4. Because of higher
osmolarity at bottom of
loop, water tends to
leave descending limb
of loop, therefore
higher osmotic conc of
tubular fluid at bottom
of loop
5. Because of high osmotic
conc of tubular fluid, NaCl
follows passively out of
ascending limb
End result-high osmotic
concentration of urea in inner
medulla interstitium is due to
urea leaving collecting duct.